Injection molding is the method of forming thermoplastic or thermoset plastic, metal, or ceramic material by injection into a closed mold. Conventionally during injection molding, an injection unit injects heated material into a closed mold device. A clamp device closes the opening in the mold in which the injection unit injects the heated material. The heated material in the mold then cools and forms, taking its intended shape. Once adequately hardened, the mold opens, the injection molded part is ejected from the mold, and the mold again closes, the clamp device opens and the next cycle begins.
During a conventional injection molding cycle the injection unit injects the material into the mold (the injection step) until the material is packed in to the mold (the pack step) and held within the mold (the hold step). The material in the mold cools (the cooling step) and plasticizes (also commonly known as plasticating) (the plasticizing step) until the material is hard enough so the mold can be opened, the material can be ejected from the mold, and the mold can close to await the next injection cycle (the open eject close step). The cooling step and the plasticizing step often occur in parallel to improve productivity.
In order to reduce the molding cycle time of injection molding machine, such machines have been provided with a valve, actuated mechanically, pneumatically or hydraulically, to cut-off communication between the mold and the plasticizing (ie. extruder) unit as soon as the plastic material has cooled enough to achieve gate freeze. The gate freeze event being defined as complete crystallization of the material at mold gate, and signifying the end of the hold step, thereby completely isolating the molded part from further influence by the plastic resin within the injection unit assembly. Once isolated, the plasticizer can begin its screw recovery and accumulate plastic material for the next shot simultaneous to the plastic material in the mold cooling sufficiently for part removal.
In the practice of injection molding, certain final part geometries are such that the cycle time may be limited by the plasticizer screw recovery time (known as a recovery limited cycle) segment rather than the typical limitation of part cooling. During recovery limited cycles a waiting period (known as the wait step) exists between the cooling and plasticizing of the first piece, while the injection molding machine prepares to inject the next volume of molten material into the mold to begin forming the next piece. When producing these types of injection molded parts, the overall machine cycle time could be improved by removing this limitation.
Various techniques have been taught to overcome this delay, notably Farrell's U.S. Pat. No. 4,070,142 and U.S. Pat. No. 4,749,536. While these modifications to a typical injection molding machine achieved both the requisite cycle time improvement and increased shot capacity, in practice these injection molding machine retrofits required extensive modifications to the injection unit machine bed and additional hydraulic pumping and valve configurations with additional control coordination.
Separate developments advanced the use of a pressurized fluid in conjunction with the plastic resin to achieve benefits of part dimensional stability, faster part cooling, stronger sections within the part via a hollow cylindrical shape, reduction of cycle time, and to reduce machine clamp tonnage. These advances were taught by Friederich originally in U.S. Pat. No. 4,101,617 as well as Hendry in U.S. Pat. No. 4,781,554 and U.S. Pat. No. 4,855,094. In these cases, the pressurized fluid is applied within the mold either by introduction through the nozzle, the runner system or directly into the mold.
Accordingly, it has been considered desirable to develop a new and improved method for injection molding and an apparatus therefore and moldings produced thereby which would overcome the foregoing difficulties and others while providing improved results.